Tape drives typically utilize an actuator mechanism to position the read/write head over the appropriate tracks while the tape is moving. Current read-write head positioning devices used in magnetic tape drives often incorporate a dual stage actuator design. One actuator provides coarse positioning to move the read-write head between data bands. The other actuator provides fine positioning to maintain alignment between the read-write head and the data tracks. In use, the coarse positioning actuator first moves the read/write head to the general vicinity on the tape and then the fine positioning actuator is used for track following while the tape is in motion. The two actuators are usually mounted in a “piggyback” arrangement with the fine position actuator riding on the coarse position actuator.
The coarse positioning actuator is typically a linear stage driven by a stepper motor. Stepper motors have the ability to move the linear stage anywhere across the width of the magnetic tape at modest speeds. However, most stepper motors lack the accuracy and bandwidth necessary to maintain alignment between the read-write head and the data tracks as the magnetic tape moves across the face of the read-write head.
The fine positioning actuator is typically a voice coil motor (VCM) mounted on the linear stage and held at a rest position by some type of spring. A VCM actuator provides micron to submicron precision positioning at a bandwidth of hundreds to thousands of hertz. However, a single voice coil and spring combination that can meet the fine positioning requirements across the full width of the tape is expensive and unnecessary. Accordingly, virtually all current tape drives use some combination of a coarse positioning actuator and a fine positioning actuator.
In typical actuator designs for tape drives, the fine VCM actuator most often utilizes a housing that must minimize the magnetic flux leakage in order to protect the read/write head from exposure to the stray bias field. This typically means that the housing must physically encapsulate the magnet and pole piece subassembly in order to achieve a minimum flux leakage. Having a housing that physically surrounds the magnet means that the hardware to connect the head to the coil must pass through the housing, often times resulting in a multiple part subassembly of the actuator system.
What is needed is a magnet configuration that minimizes the amount of pole piece material needed for the flux path, thus eliminating the need for the entire magnetic housing to control the flux path and flux leakage. What is also needed is a coil structure that takes full advantage of such a magnet configuration.